1. Field of the Invention
This invention relates to improvements in methods and apparatuses for dynamic information storage or retrieval, and more particularly to improvements in methods and circuitry for controlling the position of the data transducer, or head, used in mass data storage devices, hard disk drive devices, or the like, and still more particularly to improvements in driver circuitry and methods for use in positioning such data transducer that can be fully integrated.
2. Relevant Background
Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may-be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected to further increase in the future. Mass data storage devices may also include optical disks in which the optical properties of a spinning disk are locally varied to provide a reflectivity gradient that can be detected by a laser transducer head, or the like. Optical disks may be used, for example, to contain data, music, or other information.
The data transducer or head used in mass data storage devices is selectively positioned to desired locations of the disk by a voice coil motor (VCM). The circuitry that provides drive signals to the VCM typically has a pair of high and low side driver circuits. In operation, each set of high and low side drivers is connected on respective opposite sides of the VCM, and operate in a manner in which a high side driver from one set is activated together with a low side driver of the other set to drive a current through the VCM in one direction to move the head in a respective first direction. When the respective high and low side drivers of the opposite sides are activated, a current is driven in the opposite direction through the VCM to move the head in the opposite direction.
Typically, two transistors are used for each driver set. They provide a switching function and a sensing function. In the past, the two transistor circuit VCM driver embodiments incorporated a large single, or driving, power transistor in each side of driver set, which has been used to control both the Class-AB quiescent current and the driver output impedance. The second transistor, referred to as the sense transistor, senses the switching voltages provided by a head position control circuit, and produces a voltage to switch its associated driving transistor on or off. The driving transistor, however, is generally very large, compared to the switching transistor. In fact, in the past, the large driving transistors have been supplied as external transistors to the otherwise fully integrated driving circuit. Because of the severe mismatch between the sense transistor and the output power transistor, the sense transistor experiences a significant sub-threshold range of operation. This results in an undesirable xe2x80x9cdead zonexe2x80x9d during switching in which no current flows through the driver. This results in an undesirably high switchover harmonic distortion in the driving signals.
In the two transistors per driver side circuits, the size of the driver transistor is dictated by the output impedance design requirement. This consequently results in undesirable high Class-AB quiescent current due to the large ratio between the driving transistor and the sense (switching) transistor, as well as the design need to maintain an acceptable level of switchover harmonic distortion. This high current results in disadvantageous power dissipation in the drivers during the Class-AB operation.
What is needed therefore, is a means for controlling the output impedance of a VCM driver circuit, as well as controlling its Class-AB operation and associated harmonic content with a circuit that ideally can be fully integrated into a single driver circuit.
In light of the above, therefore, it is an object of the invention to provide a Class-AB driver circuit for a VCM of a mass data storage device in which better control and predictability of the Class-AB quiescent current can be achieved.
It is another object of the invention to provide a Class-AB driver circuit of the type described in which the output impedance of the drivers can be modified, if required, without a significant impact to performance and stability of the circuit.
It is another object of the invention to provide a Class-AB driver circuit of the type described in which predictable power consumption and crossover linearity of the VCM current can be achieved.
It is still another object of the invention to provide a Class-AB driver circuit of the type described that has lower harmonic switchover distortion.
It is still yet another object of the invention to provide a Class-AB driver circuit of the type described that can be fully integrated.
As will become apparent from the description below, one of the advantages of the circuit of the invention is that all of the transistors of each driver section can be, and preferably are, fully integrated onto a single silicon wafer.
It is another advantage of the invention to provide a circuit of the type described in which the Class-AB operation can be maintained and controlled during switchover, without the occurrence of discontinuities thereat.
It is another advantage of the invention that possible subthreshold operation of the switching transistors can be avoided or eliminated.
It is another advantage of the invention that the total harmonic distortion of the circuit can be greatly reduced due to the elimination of any discontinuities at the Class-AB switchover point. It has been found, for example, that operation at frequencies as high as 50 KHz can be achieved with harmonic distortion in the region of 0.6%, or less.
It is yet another advantage of the invention that the system incorporating circuits constructed in accordance with the invention can be operated at much higher frequencies, for example on the order of 50 KHz without significant harmonic distortion.
It is another advantage of the invention that the power dissipation of the circuit during switchover is greatly reduced from previous circuits.
Yet another advantage is that circuits constructed in accordance with the invention are flexible in enabling size modifications of the output circuit, without requiring a complete circuit redesign.
These and other objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.
According to the present invention, instead of having one power transistor to control both the Class-AB quiescent current and the driver output impedance, each of those design parameters is independently controlled by a separate power transistor.
According to a broad aspect of the invention, a circuit is presented for providing drive voltages to a voice coil motor (VCM) of a mass data storage device. The circuit has two driver sets, each including a high side driver and a low side driver, for connection to respective opposite sides of the VCM. Each driver includes an output FET to selectively connect an output node connected to the VCM to a first reference potential and a quiescent current supply circuit in parallel with the output FET to maintain a continuous quiescent current in the driver set. The bias circuit may include a quiescent current controlling FET connected in parallel with the output FET and a biasing FET connected to bias the quiescent current controlling FET, the biasing FET being connected at a first side to the output node. In order to limit power dissipation in the drivers and maintain a low level of crossover harmonic distortion, the output FETs are turned off around the crossover point. As a result, the amplifier characteristics (open-loop gain, unity gain-bandwidth, slew-rate, etc.) at the quiescent operating point are controlled independently of the output FET characteristics. Preferably the output FET, the quiescent current controlling FET, and the biasing FET are fully integrated in a single integrated circuit.
According to another broad aspect of the invention, a circuit is presented for providing drive voltages to a voice coil motor (VCM). The circuit has two sets of high and low side drivers, each set for connection to respective opposite sides of the VCM. Each set of the high and low side drivers has first and second driver FET to respectively pull up and pull down an output node connected to the VCM. First and second circuits are connected in parallel respectively with the first and second driver FETs. Each of the first and second circuits is constructed to provide a continuous quiescent current in the driver set when the driver FETs are not conducting. Each of the first and second circuits includes a quiescent current controlling FET connected in parallel with a respective associated the driver FET and a biasing FET connected to bias the quiescent current controlling FET, while the respective the driver output FET is not conducting. Preferably, the driver FET, the quiescent current controlling FET, and the biasing FET are fully integrated in a single integrated circuit.
According to yet another broad aspect of the invention, a mass data storage device is presented. The mass data storage device is of the type having a circuit for providing drive voltages to a voice coil motor (VCM) of a mass data storage device. The circuit has two circuit sets, each including a high side driver and a low side driver for connection to respective opposite sides of the VCM. Each driver has an output FET to selectively connect an output node connected to the VCM to a first reference potential and a quiescent current supply circuit in parallel with the output FET to provide a continuous quiescent current in the Class-AB output stage of each set at the crossover point, while the output FET transistor is kept not conducting. The bias circuit has a quiescent current controlling FET connected in parallel with the output FET and a biasing FET connected to bias the quiescent current controlling FET, while the output FET is not conducting, the biasing FET being connected at a first side to the output node. Preferably, the output FET, the quiescent current controlling FET, and the biasing FET are fully integrated in a single integrated circuit.
According to yet another broad aspect of the invention, a method is presented for providing drive voltages to a voice coil motor (VCM) of a mass data storage device of the type having two circuit sets, each including a high side driver and a low side driver for connection to respective opposite sides of the VCM to alternatively pull up and pull down the respective opposite sides of the VCM. The method includes switchably applying an output drive current to the VCM and supplying a constant quiescent current to the output stage when the output drive current is crossing over. The continuous quiescent current is supplied to the Class-AB output stage to maintain linearity of operation thereof. The output drive current is switchably applied to the VCM by providing in each of the high and low side drivers a drive transistor to switchably connect the VCM to a reference potential. The continuous Class-AB current is supplied to the VCM by providing in each of the high and low side drivers a Class-AB transistor pair that is much smaller than and in parallel with the drive transistor pair. Each transistor of the Class-AB pair is biased to turn on at a lower bias voltage than the drive transistor. Preferably, the drive, Class AB and bias transistors are all integrated in a single integrated circuit.